1. Field of the Invention
This invention relates to a variable turbine geometry turbocharger for an internal combustion engine. More particularly, this invention relates to an asymmetric bushing for an actuator pivot shaft.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine having a turbine housing connected to the engine's exhaust manifold, a compressor having a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. The turbine includes a turbine wheel disposed within the turbine housing and the compressor includes a compressor impeller disposed within the compressor housing. The turbine wheel is rotatably driven by a flow of exhaust gas supplied from the exhaust manifold. A shaft is rotatably supported in the bearing housing and couples the turbine wheel to the compressor impeller such that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines a turbocharger axis of rotation. As the compressor impeller rotates, it compresses ambient air entering the compressor housing, thereby increasing the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
To improve efficiency, responsiveness, or the operating range of turbochargers, it is often advantageous to regulate the flow of exhaust gas to the turbine wheel. One method of regulating the flow of exhaust gas to the turbine wheel is commonly referred to by several names, including Variable Turbine Geometry (VTG), Variable Geometry Turbine (VGT), Variable Nozzle Turbine (VNT), or simply Variable Geometry (VG). VTG turbochargers include a plurality of adjustable guide vanes pivotally supported within a wheel inlet leading to the turbine wheel. The guide vanes are adjusted to control exhaust gas back pressure and the turbocharger speed by modulating the flow of exhaust gas to the turbine wheel.
For example, adjusting the guide vanes to constrict the flow of exhaust gas increases the velocity of the exhaust gas impacting the turbine wheel, which causes the turbine wheel to rotate more quickly. Increasing the rotation of the turbine wheel in turn increases the rotation of the compressor impeller, and thereby increases the boost pressure delivered to the engine. Conversely, adjusting the guide vanes to open the flow of exhaust gas decreases the velocity of the exhaust gas impacting the turbine wheel, which causes the turbine wheel to rotate more slowly. Decreasing the rotation of the turbine wheel in turn decreases the rotation of the compressor impeller, and thereby decreases the boost pressure delivered to the engine.
Adjusting the guide vanes also provides a means for generating and controlling exhaust gas back pressure in engines which use Exhaust Gas Recirculation (EGR) to control Nitrogen Oxide (NOx) emissions.
Referring to FIGS. 1A to 3, various elements of a typical VTG turbocharger 100 are shown. The turbocharger 100 includes a bearing housing 102 and defines a turbocharger axis of rotation R10. A vane assembly includes a plurality of guide vanes 104 mounted on vane shafts 105 between a lower vane ring 106 and an upper vane ring 108. The vane shafts 105 are rotated to adjust the position of the guide vanes 104. A vane lever or vane fork 110 is fixedly secured to an end of the vane shaft 105. The vane forks 110 engage vane blocks 114 that are rotatably coupled to a control ring 116. The guide vanes 104 are rotatably driven by the vane forks 110 in response to rotation of the control ring 116 in first and second directions about the turbocharger axis of rotation R10. The control ring 116, in turn, is rotated by an actuator pivot shaft 118.
The actuator pivot shaft 118 extends from outside the turbocharger 100 into the bearing housing 102. Attached to an inside end 120 of the actuator pivot shaft 118 is a pivot shaft fork 122. Displacement of a control linkage 124 by an actuation device (not shown) rotates a pivot arm 126 attached to the actuator pivot shaft 118 outside the bearing housing 102. The displacement of the control linkage 124 results in a rotation of the actuator pivot shaft 118 about its axis R12. This rotation of the actuator pivot shaft 118 is carried inside the bearing housing 102 and translates into rotation of the pivot shaft fork 122. The rotation of the pivot shaft fork 122 acts on an actuator block 130 that is rotatably coupled to the control ring 116, which results in rotation of the control ring 116 and corresponding adjustment of the guide vanes 104, as described above.
According to the prior art, the actuator pivot shaft 118 is supported by two bushings. A primary bushing 132 is located in a bore 134 through which the actuator pivot shaft 118 extends into the bearing housing 102. The primary bushing 132 provides radial constraint for the actuator pivot shaft 118 adjacent the pivot shaft fork 122. A secondary bushing 136 is located in a bore 138 in an outer portion of the bearing housing 102 and provides radial constraint for an outside end 140 of the actuator pivot shaft 118.
The fit between the actuator pivot shaft 118 and the primary bushing 132 allows for rotation of the actuator pivot shaft 118 without binding. Thus, when the actuation device is operated to adjust the guide vanes 104, there often are non-rotational forces that may tilt or skew the actuator pivot shaft 118. These non-rotational forces cause high contact pressure between the actuator pivot shaft 118 and the primary bushing 132, which leads to premature wear of the actuator pivot shaft 118.
It is desirable, therefore, to provide an improved bushing to prevent tilting of an actuator pivot shaft, thereby reducing contact pressure between the actuator pivot shaft and the bushing.